Vacuum valves, as used in both aspects of the invention, according to the prior art are often controlled in dependence upon the position of a pressure or casting piston (or other pressure member) with a certain time delay so that they close in time to avoid escape of material through the valve. Examples of such control systems can be found in U.S. Pat. Nos. 2,837,792; 2,904,861; 3,349,833; 4,463,793 or 4,577,670. It is clear that the position of the pressure member gives merely an approximate indication where the front of material (the metal in the case of a die-casting machine or the plastic material in the case of an injection molding machine) actually is. That is for the actual position of the front of material will also depend upon the filling degree or degree of admission within the shot-sleeve or within the extruder barrel and may vary with fluctuation of the dosed and supplied amount of material.
This unsatisfactory condition could be solved by assigning to the vacuum valve at least one sensor of a type which is able to react quickly enough to determine arrival of the front of material in the region of the cavity, particularly near the vacuum valve, and to close the vacuum valve in time. Recently, such quickly reacting sensors have been developed.